Free-floating planets are planetary-mass objects that do not orbit a star, but roam the galaxy isolated. Apart from micro-lensing detections, only a few tens of directly imaged free-floating planet candidates are known to date [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]} and only a small fraction have been confirmed so far. Because of the degeneracy in the mass-luminosity relationship for these ultracool objects, it is indeed impossible to distinguish a low mass brown dwarf from a planetary mass object when the age and distance are unknown. This deadlock can be overcome by studying free-floating planets members of young associations where the age and distance are precisely known. In the context of the COSMIC-DANCEproject-dance.com survey [9]} obtained deep optical and near-infrared photometry and measured accurate proper motions 5 mag beyond Gaia's limit in the nearby Upper Scorpius OB association (USco) and \(\rho \) -Ophiuchus (Oph) molecular clouds. They identified over 3 500 members, including between 70 and 170 planetary mass objects, depending on the age assumed. This large number of planetary mass object candidates in a young association has important implications for the theories and models of star, brown dwarf and planet formation. In order to confirm this important discovery, we performed follow-up spectroscopic observations of 18 free-floating planet candidates to confirm their nature and membership to the association and validate [9]} analysis. In the following, we describe the observations and the processing of the data obtained at the Grantecan and Subaru telescopes. We discuss the membership to the association by looking for spectral features characteristic of young ultracool objects. Finally, we estimate the spectral types of the objects and derive effective temperature and mass estimates, as well as a contamination rate in [9]} sample.
